1. Field of the Invention
The present invention relates generally to wafer processing, and more particularly to methods and apparatus for adjusting the lithium oxide concentration in wafers.
2. Description of the Background Art
Lithium tantalate (LiTaO3) and lithium niobate (LiNbO3) are widely used materials for fabricating nonlinear optical devices because of their relatively large electro-optic and nonlinear optical coefficients. These nonlinear optical devices include wavelength converters, amplifiers, tunable sources, dispersion compensators, and optical gated mixers, for example.
Stoichiometric lithium tantalate (SLT) and congruent grown lithium tantalate (CLT) are two types of lithium tantalate wafers. An example of a lithium niobate wafer is the so-called congruent grown lithium niobate (CGN). It has been shown that SLT has better lifetime and ferroelectric properties than CLT and CGN in nonlinear optical devices; e.g., see “Crystal Growth and Low Coercive Field 180° Domain Switching Characteristics Of Stoichiometric LiTaO3,” Applied Physics Letters, Nov. 23, 1998, Vol. 73, Number 21, by K. Kitamura et al. However, although SLT has desirable properties, SLT wafers are relatively difficult to obtain. In contrast, CLT wafers are produced in large quantities by commercial suppliers and are thus widely available.
One way of fabricating SLT wafers is by the vapor transport equilibration method described in U.S. Pat. Nos. 4,071,396 and 4,071,323, which are both issued to Robert L. Holman (“Holman”). U.S. Pat. Nos. 4,071,396 and 4,071,323 are incorporated herein by reference in their entirety. In Holman, a target wafer is exposed to lithium oxide (Li2O) vapor produced by heating a mass of a lithium-rich two-phase powder. The two-phase powder produces a constant vapor pressure of lithium oxide equal to the vapor pressure of stoichiometric lithium tantalate. The wafer, which is initially deficient in lithium oxide, absorbs lithium oxide from the vapor until it reaches the stoichiometric composition (i.e., lithium oxide concentration of 50 mol %, tantalum pentoxide concentration of 50 mol %). At that point, the vapor pressure of stoichiometric lithium tantalate over the surface of the wafer equals the vapor pressure of the surrounding lithium oxide, thereby reaching a process equilibrium and stopping the diffusion of lithium oxide into the wafer.
The aforementioned Holman process has several disadvantages. One disadvantage is that the volume of the two-phase lithium-rich powder may be greater than that of the target wafer. This may limit the throughput of commercially available furnaces for performing the process in that there will be less available process tube flat zone left available for wafers. Another disadvantage is that the wafers are placed in close proximity to a large amount of crumbly two-phase lithium-rich powder, increasing the potential for surface contamination. Still another disadvantage of the Holman process is that it requires a space-inefficient containment vessel to eliminate pressure gradients. Yet another disadvantage of the Holman process is that it restricts the resulting lithium oxide concentration in the wafer to be that of stoichiometric lithium tantalate. Although there are applications where a stoichiometric composition is desirable, the Holman process cannot be used in other applications where the lithium oxide concentration in the wafer is preferably below 50 mol %.
SLT wafers can also be fabricated using the double-crucible Czochralski (DCC) growth method. In the DCC growth method, a boule of lithium tantalate is pulled from a melt in the center crucible of a concentric crucible pair. The lithium oxide concentration in the melt is chosen such that the initially grown material is of the stoichiometric composition. As the boule grows and is pulled from the melt, a stoichiometric mixture of lithium oxide and tantalum pentoxide (Ta2O5) powder is poured into the outer crucible at a rate carefully controlled to equal the rate of crystal growth.
Crystal growth rate using the DCC growth method is a fraction of the growth rate achievable using congruent growth methods. Thus, SLT wafers fabricated using the DCC growth method are not as cost effective as CLT wafers. Also, in the DCC growth method, striations in the resulting wafers are difficult to suppress, causing variations in optical properties from wafer to wafer.
Another way of fabricating SLT wafers is by the Czochralski growth from a lithium-rich melt (LRM) method. In the LRM growth method, only a fraction (e.g., approximately 10%) of the melt is used to produce the stoichiometric boule. Because continued growth after using the fraction of the melt results in rapid deviation from the stoichiometric composition, the melt is frequently recycled. Tantalum pentoxide powder is added to the recycled melt to achieve the appropriate lithium oxide to tantalum pentoxide concentration for the next growth run.
The LRM growth method grows material at a slower rate than congruent growth methods, and has the additional throughput reduction associated with the time required to recycle the used melt. Further, because accurate measurement of lithium oxide concentration in a used melt is difficult, approximations are made to determine the lithium oxide concentration in the melt. This results in variations in the amount of useful grown material from boule to boule.
From the foregoing, an improved technique for adjusting the lithium oxide concentration in wafers is highly desirable. Ideally, such a technique should also allow production of SLT wafers in large quantities and at a relatively low cost.